The following account of the prior art relates to one of the areas of application of the present invention.
Electronic hearing devices, such as hearing aids, listening devices and ear protection devices, are well known in the art. Hearing aids and listening devices known in the prior art are typically small devices intended to be placed in, at or near the person's ear. Such devices may be categorized according to their placement, e.g. behind-the-ear (BTE), in-the-ear (ITE), in-the-ear-canal (ITC), completely-in-the-canal (CIC) or receiver-in-the-ear (RITE). In most cases, it is desirable that the hearing device be small and light-weight in order to improve the comfort of wearing. Ear protection devices may similarly be placed close to or within the ear canal, and should for the same reason be small and light-weight.
Known hearing devices typically comprise a main microphone, a receiver and a signal conditioning means connected to both the main microphone and the receiver. The main microphone receives acoustical input signals from the person's surroundings and converts these into electrical input signals, which it feeds to the signal conditioning means. The signal conditioning means modifies, e.g. amplifies, attenuates and/or filters, the electrical input signals and feeds the resulting electrical output signals to the receiver, which converts the electrical output signals into acoustical output signals and transmits these into the ear and/or the ear canal. In modern day hearing devices, the signal conditioning means typically comprises analog-to-digital and digital-to-analog converters and performs the signal conditioning digitally. Known receivers typically comprise an electromagnetic loudspeaker, the acoustically radiating body of which comprises a diaphragm driven by a permanent magnet, which moves relative to an electrically driven coil, or vice versa.
Hearing devices which are intended for partial or complete placement in the ear canal—or at the canal's opening into the outer ear, are typically designed to close the ear canal completely in order to create a defined acoustical chamber within the ear canal. However, an air-tight closing of the ear canal causes a discomfort known as occlusion. In order to avoid this, known hearing devices of this type are typically provided with a vent, which connects the ear canal with the ambient air. In the case that the hearing device comprises an ear plug for insertion into the ear canal, the vent is typically formed as a tubular channel extending through the ear plug.
The receiver radiates the acoustical signals into the ear and/or the ear canal, either directly or indirectly e.g. via a tube. Normally, it is desired to have well-defined signal amplification gains between the acoustical input signals received by the main microphone and the acoustical signals presented to the tympanum. However, the actual sound pressure levels at the tympanum depend not only on the sound pressure levels radiated by the receiver, but also on the acoustical impedances of the passage and/or tube leading from the receiver to the ear canal and of the acoustical chamber created within the ear canal. These impedances are often not known precisely and may further change with position and orientation of the hearing device relative to the ear and/or ear canal. Thus, the sound pressure level at the tympanum may vary. In order to allow for producing a more precise sound pressure level at the tympanum, the hearing device may be equipped with a monitoring microphone, which is arranged so that it receives acoustical signals from the chamber in the ear canal. The signal conditioning means may use the signals received by the monitoring microphone to modify the signals transmitted to the receiver in a manner suited to maintain a desired amplification gain. Such signal modifications may take place in various ways of which several are known in the art.
Depending on the configuration of the hearing device, mechanical vibrations induced by the diaphragm and/or other moving parts of the receiver may undesirably be fed back to the main microphone. The feedback may occur as acoustical feedback, e.g. through the vent, as mechanical feedback through the structure of the hearing device and/or as a combination of both, e.g. through the bone structure of the wearer and the ambient air. At large amplification gains, the feedback may cause the hearing device to howl or whistle, which may be very annoying for the wearer. In order to reduce the tendency to howl or whistle at large amplification gains, known hearing devices typically implement one or more methods for cancelling the feedback signal. A well known method comprises the steps of adaptively estimating the feedback signal on the basis of the signals presented to the receiver, subtracting the estimated feedback signal from the signal received by the main microphone, and using the resulting signal as input for the signal conditioning means. Alternatively, the signal conditioning means may e.g. reduce the amplification gain when it detects the presence of whistling or howling, and/or when it detects a situation in which the risk thereof has increased.
The signal conditioning means typically comprises an output stage for driving the receiver. In modern day hearing devices, the output stage typically comprises a so-called class D output amplifier, which switches its output between a positive and a negative voltage, thereby producing square-wave output signals. The switching typically takes place at a frequency at the upper end of or above the audible frequency range, and the switching signals are modulated to produce the desired output signals in the audible frequency range. The coil and magnet of the receiver typically serve as a low-pass filter to suppress undesired high frequency components of the square-wave output signals.
In their paper, “Flexible, Stretchable, Transparent Carbon Nanotube Thin Film Loudspeakers”, published by The American Chemical Society on pp 4539-4545 of “Nano Letters 2008, 8 (12)”, with the web publication date of Oct. 29, 2008, Lin Xiao et. al describe a loudspeaker formed from a carbon nanotube thin-film.